Sculk mass is used by the horde to summon units on the battlefield. Every unit cost mass to create, by killing Sculk Horde mobs, you are permanently removing resources from the horde. If a Sculk Horde mob despawns, this mass is recycled to the horde.
Used to spawn Sculk Nodes in the world. It cost a large amount of mass to spawn these node structures.
It is also used for evolving the Gravemind. The Gravemind grows stronger the more mass it has accumulated.